Method for determining an inductance of a motor

ABSTRACT

The intention is to simplify the measurement of an inductance characteristic curve of a motor. To this end, provision is made for a current (i) having a non-periodic current offset component and a periodic current component to be injected into the motor&#39;s winding so that the motor accelerates. A corresponding voltage (u) across the winding is measured and a voltage interference component and a periodic voltage component are determined from the measurement. The inductance of the winding can finally be determined from these two components. It is thus possible to dispense with the operation of blocking the motor or to dispense with an expensive motor test rig in order to determine the inductance characteristic curve.

The present invention relates to a method for determining an inductanceof a winding of an electric motor.

In order to adapt a current regulator of an electric motor, it isgenerally necessary to determine accurately the inductances and/orinductance profiles of the motor. Since the inductances are not constantvariables but, inter alia, are current-dependent variables,correspondingly complex measurements are required. Until now, in orderto measure a so-called q-inductance line, the motor has had to bestalled or has had to be kept at a constant rotation speed on a motortest rig. However, there are frequently no facilities to do this onsite, since suitable test equipment is expensive. The measurement of theso-called q-inductance therefore plays a major role for currentregulator adaptation because this inductance decreases as the currentrises, thus requiring adaptation in the current regulator.

The object of the present invention is therefore to simplify the processof determining an inductance characteristic of an electric motor.

According to the invention, this object is achieved by a method fordetermining an inductance of a winding of an electric motor by passingcurrent through the winding with a non-periodic current offset componentand a periodic current component, such that the motor accelerates, byproviding a voltage signal for the winding and determining a voltagedisturbance component and a periodic voltage component from this, anddetermining the inductance of the winding from the periodic currentcomponent or a measured, periodic current signal and the periodicvoltage component.

Alternatively, the object mentioned above is achieved with the aid of amethod for determining an inductance of a winding of an electric motorby application of a voltage to the winding with a non-periodic voltageoffset component and a periodic voltage component, such that the motoraccelerates, by providing a current signal for a current through thewinding and determining a current disturbance component and a periodiccurrent component from this, and determining the inductance of thewinding from the periodic current component and the periodic voltagecomponent or a measured, periodic voltage signal.

The invention is based on the discovery that it is better to carry outthe inductance measurement during acceleration since neither a stallingfacility nor a motor test rig is often available. The aim is thereforeto allow the q-inductance (inductance relating to the torque-formingcurrent) to be measured during acceleration of the motor. For thispurpose, a superimposed, sinusoidal alternating signal (torque-formingcurrent) is applied to a q-current offset, and the Fourier coefficientsof the associated signals are determined from the current actual valueand the voltage actual value for the respective sinusoidal frequency.The q-inductance can be calculated from these coefficients. However, theq-voltage rises during the acceleration, so that there is adiscontinuous transition between the initial value and the final value.

Discrete Fourier transformation or fast Fourier transformation (DFT orFFT) is normally used to determine the Fourier coefficients. In thiscase, the measured signals are implicitly continued periodically, andthe frequency components of this signal produced in this way aredetermined. If a discontinuous transition occurs in this case betweenthe initial value and the final value, the result is highly dominated bythis discontinuity. Exact determination of the frequency components ofthis signal would not be possible from the DFT or FFT on its own. Inorder nevertheless to allow the frequency components to be determineddespite these discontinuities, non-periodic disturbance components inthe measured signal are estimated or determined. Only the periodiccomponents of the signal are then used to determine the inductance.

The current or voltage disturbance components can also be estimated wellby means of a polynomial, in particular a second-order polynomial. Thismakes it possible to separate the non-periodic component of the signalssubstantially from the periodic component.

According to one particularly preferred embodiment of the presentinvention, Fourier coefficients of the periodic current component and ofthe periodic voltage component are calculated in order to determine theinductance. This allows the specific profile of the inductance to becalculated very exactly.

The present invention will now be explained in more detail withreference to the attached drawing, which illustrates the current andvoltage profile on a motor, in order to determine its inductance.

The exemplary embodiment described in more detail in the following textrepresents one preferred embodiment of the present invention.

In order to measure an inductance of a motor, a current i is applied tothe motor, as is illustrated in the FIGURE. This current contains anon-periodic component and a periodic component. In this specific case,the sinusoidal component has a continuously rising offset component(actual value) superimposed on it, because of the acceleration. However,in principle, a constant offset component (nominal value) is desired.

In this specific example, the measured voltage u has the profile shownin the FIGURE. This is also characterized by a sinusoidal component onwhich a non-periodic component in the form of a ramp is superimposed.The non-periodic component of the voltage u in this case rises moresharply than the non-periodic part of the current i.

The measured current and voltage signals (actual values) can berepresented by a signal model which models the expected disturbances(caused inter alia by the acceleration of the motor) and the sinusoidalsignals. A signal model with a sine, cosine and a second-orderpolynomial can be used as one specific example. The polynomial containsthe rising voltage (disturbance) during the acceleration of the motor.The factors in front of the sine and the cosine correspond to the soughtFourier coefficients when the disturbance model is removed from thesignal (in this case the polynomial). The signal model can be appliedfor the current signal iq(t) and the voltage signal uq(t) as follows:

iq(t)=ki ₀ +ki ₁ ·t+ki ₂ ·t ² +rei·cos(ω· t)+imi·sin(ω·t)

uq(t)=ku ₀ +ku ₁ ·t+ku ₂ ·t ² +reu·cos(ω· t)+imu·sin(ω·t)

The aim is to calculate the inductance from these equations and thebasic equation

$Z = {{j\; \omega \; L} = {\frac{U}{I}.}}$

This is done by determining the coefficients rei, imi, reu and imu. Inprinciple, these coefficients can also be obtained from nominal andactual values of the current and voltage signals.

In order to calculate all the coefficients of the signal model, moremeasurement points must be obtained than there are coefficients. In thepresent example, more than five measurement points must be determined inorder to determine the coefficients of the current profile. However,considerably more measurement points will normally be determined, sincethe signals are generally noisy.

The above equations can be solved, for example, using the method ofminimizing the error squares (Gaussian algorithm). In this case, inparticular, the coefficients in front of the cosine and the sine are ofinterest because they correspond to the Fourier coefficients by means ofwhich the inductance can be calculated.

The coefficients in the stated equations can even be calculated on-line,that is to say during the measurement, because of the comparativelysmall amount of computation complexity. If required, the necessarymatrix inversion for calculation of the sought coefficients can becarried out in advance, that is to say off-line, and appropriateconstants can be stored for calculation.

Sine components in measurement signals which cannot be continuedperiodically can therefore advantageously be determined in order todetermine the inductance profiles. Specifically, the q-inductancecharacteristic can also be measured during acceleration of the motor. Inconsequence, there is no need to provide any additional mechanicalcomponents, such as a motor test rig or stalling device, for ameasurement such as this. This type of motor data identificationconsiderably simplifies motor commissioning.

1.-5. (canceled)
 6. A method for determining an inductance of a windingof an electric motor comprising the steps of: passing a current signalthrough the winding such that the motor accelerates, said current havinga non-periodic current offset component and a periodic currentcomponent; determining a voltage disturbance component and a periodicvoltage component of a voltage signal produced when the current isapplied to the winding; and calculating an inductance of the windingusing the periodic current component and the periodic voltage component.7. The method of claim 6, wherein the periodic current component is ameasured periodic current signal.
 8. The method of claim 6, wherein thecurrent disturbance component satisfies a polynomial.
 9. The method ofclaim 8, wherein the current disturbance component satisfies asecond-order polynomial.
 10. The method of claim 6, wherein the voltagedisturbance component satisfies a polynomial.
 11. The method of claim10, wherein the voltage disturbance component satisfies a second-orderpolynomial.
 12. A method for determining an inductance of a winding ofan electric motor comprising the steps of: applying a voltage signal tothe winding such that the motor accelerates, said voltage having anon-periodic voltage offset component and a periodic voltage component;determining a current disturbance component and a periodic currentcomponent of a current signal produced when the voltage is applied tothe winding; and calculating an inductance of the winding using theperiodic current component and the periodic voltage component.
 13. Themethod of claim 12, wherein the periodic voltage component is a measuredperiodic voltage signal.
 14. The method of claim 12, wherein the currentdisturbance component satisfies a polynomial.
 15. The method of claim14, wherein the current disturbance component satisfies a second-orderpolynomial.
 16. The method of claim 12, wherein the voltage disturbancecomponent satisfies a polynomial.
 17. The method of claim 16, whereinthe voltage disturbance component satisfies a second-order polynomial.18. The method of claim 6, wherein the step of determining a voltagedisturbance component and a periodic voltage component of the voltagesignal produced when the current is passed through the winding includescalculating a Fourier coefficient of the periodic voltage component,said method further comprising the step of calculating a Fouriercoefficient of the periodic current component for use in calculating theinductance.
 19. The method of claim 6, wherein the step of determiningan inductance of the winding uses coefficients of the periodic currentcomponent and the periodic voltage component, and the step ofdetermining an inductance of the winding further comprises the steps ofdetermining and storing constants of a matrix inversion used tocalculate coefficients of the periodic current component and theperiodic voltage component, and using the stored constants to calculatethe coefficients of the periodic current component and the periodicvoltage component used to calculate the inductance of the winding. 20.The method of claim 12, wherein the step of determining a currentdisturbance component and a periodic current component of the currentsignal produced when the voltage is applied to the winding includescalculating a Fourier coefficient of the periodic current component,further comprising the step of calculating a Fourier coefficient of theperiodic voltage component for use in calculating the inductance. 21.The method of claim 12, wherein the step of determining an inductance ofthe winding uses coefficients of the periodic current component and theperiodic voltage component, and the step of determining an inductance ofthe winding further comprises the steps of determining and storingconstants of a matrix inversion used to calculate coefficients of theperiodic current component and the periodic voltage component, and usingthe stored constants to calculate the coefficients of the periodiccurrent component and the periodic voltage component used to calculatethe inductance of the winding.